While fiber-optic cable is finding widespread use for data transmission and other telecommunication applications, the relatively high cost of new installed fiber-optic cable presents a barrier to increased carrying capacity. Wavelength division multiplexing (WDM) allows multiple different wavelengths to be carried over a common fiber-optic waveguide. Presently preferred wavelength bands for fiber-optic transmission media include those centered at 1.3 micrometer and 1.55 micrometer (micrometer is written ".mu.m" below). The latter, with a useful bandwidth of approximately 10 to 40 nm depending on the application, is especially preferred because of its minimal absorption and the commercial availability of erbium doped fiber amplifiers. Wavelength division multiplexing can separate this bandwidth into multiple channels. Dividing bandwidth into multiple discreet channels, such as 4, 8, 16 or even as many as 32 channels, through a technique referred to as dense channel wavelength division multiplexing (DWDM), is a relatively lower cost method of substantially increasing telecommunication capacity, using existing fiber-optic transmission lines. Thus, wavelength division multiplexing may be used in a fiber-optical telecommunication system supplying voice and data transmission, as well as video-on-demand and other existing or planned multimedia, interactive services. Techniques and devices are required, however, for multiplexing the different discreet carrier wavelengths. That is, the individual optical signals must be combined onto a common fiber-optic line or other optical waveguide and then later separated again into the individual signals or channels at the opposite end or other point along the fiber-optic cable. Thus, the ability to effectively combine and then separate individual wavelengths (or wavelength sub-ranges) from a broad spectral source is of growing importance to the fiber-optic telecommunications field and other fields employing optical instruments.
Optical multiplexers are known for use in spectroscopic analysis equipment and for the combination or separation of optical signals in wavelength division multiplexed fiber-optic telecommunications systems. Known devices for this purpose have employed, for example, diffraction gratings, prisms and various types of fixed or tunable filters. Gratings and prisms typically require complicated and bulky alignment systems and have been found to provide poor efficiency and poor stability under changing ambient conditions. Fixed wavelength filters, such as interference coatings, can be made substantially more stable. In this regard, highly improved interference coatings of metal oxide materials, such as niobia and silica, can be produced by commercially known plasma deposition techniques, such as ion assisted electron beam evaporation, ion beam sputtering, reactive magnetron sputtering, e.g., as disclosed in U.S. Pat. No. 4,851,095 to Scobey et al and in U.S. Pat. No. 5,525,741 to Scobey. Such coating methods can produce interference cavity filters formed of stacked dielectric optical coatings which are advantageously dense and stable, with low film scatter and low absorption, as well as low sensitivity to temperature changes and ambient humidity. The theoretical spectral performance of a stable, three-cavity tilted filter (tilted, for example, at about 8.degree. from normal) produced using such advanced deposition methods is shown in FIG. 1 of the appended drawings. The spectral profile of FIG. 1 shows transmission through a tilted filter element resulting in polarization splitting of the signal. Polarization splitting can results in polarization dependent loss (PDL), that is, differential signal loss for the P-polarization and the S-polarization components or states of the signal. It will be understood that a higher tilt angle results in greater polarization splitting and, therefore, can result in correspondingly higher PDL. However, high performance, multi-cavity filters (for example, 3 to 5 cavity Fabry-Perot type interference filters wherein the film stack is formed of deposited films of near unity density) yield a flat in-band transmission zone, as shown in FIG. 1. This reduces polarization dependent loss, because the two polarization states overlap in-band. A high performance filter here means one providing such a flat in-band transmission zone and correspondingly low (for example, less than 1 dB, preferably less than 1/2 dB) insertion loss. In optical multiplexing devices intended for the teleconmmunications industry, preferably there is as little polarization dependent loss as possible in the optical signal path. The filter performance shown in FIG. 1 is seen to be suitable to meet stringent telecommunication system specifications.
Alternative approaches for selectively removing or tapping a channel, i.e., selective wavelengths, from a main trunk line carrying multiple channels, i.e., carrying optical signals on a plurality of wavelengths or wavelength sub-ranges, is suggested, for example, in U.S. pat. No. 4,768,849 to Hicks, Jr. In that patent filter taps are shown, as well as the use of gangs of individual filter taps, each employing high performance, multi-cavity dielectric pass-band filters and lenses for sequentially removing a series of wavelength sub-ranges or channels from a main trunk line. The filter tap of Hicks returns a multi-channel signal to the main trunk line as it passes the desired channel to a branch line. Optical multiplexing devices are shown also in U.S. Pat. No. 4,244,045 to Nosu et al, for multiplexing or demultiplexing a multi-channel optical signal. A row of individual optical filters are glued side-by-side onto the surface of an optical substrate, and a second row is glued similarly to the opposite surface of the substrate. Each individual filter transmits a different channel, that is, a preselected wavelength(s), and reflects other wavelengths. A multi-channel optical beam from a trunk line enters the optical substrate at an angle and passes thru the substrate from filter to filter in a zig-zag fashion. Each filter transmits its preselected wavelength(s) and reflects the remainder of the beam on to the next filter. Each filter element is sandwiched between glass plates, and a prism element is positioned between each filter sandwich and a corresponding collimator positioned behind the filter sandwich to receive the transmitted wavelength(s). Nosu et al teaches the use of refractive index matching. The lenses, filters, optical substrate, etc. all have the same refractive index and are in surface-to-surface contact with one another, such that the light beam does not pass through air. This approach by Nosu et al involves the use of prisms as an optical bridge between the filter element and the collimater at each channel outlet. This elaborate design approach adds considerable cost and assembly complexity to multiplexing devices of the type shown in Nosu et al. The quite wide (20 nm half width) and undesirably "peaky" pass-band of FIG. 5 of Nosu et al ("peaky" here meaning not having a flatter top for the pass-band, such as in FIG. 1 of the drawings appended hereto) are characteristic for the mono-cavity filters taught by Nosu et al and shown there in FIG. 4. Devices with such pass-band characteristics as in Nosu et al have higher polarization dependent losses (PDL) as discussed above. In that regard, FIG. 5 of Nosu et al must be understood to represent an average of the actual S-polarization and P-polarization pass-bands which would be shown more accurately as being slightly offset from each other. With such a peaky pass-band, any slight change in the signal wavelength (e.g., due to normal system instability or variability) will result in undesirably high PDL by unequally effecting the S-polarization component of the signal and its P-polarization component.
Devices such as those of Nosu et al employ adhesive in the light path, e.g., to adhere the prisms to the collumations and filter sandwiches. This is undesirable for several reasons. There is uncertainty as to the long term (e.g., 10 years) durability or stability of adhesives in such devices. The transparency of epoxy or other adhesive may change after exposure to thermal cycling, etc. Also, such "glue in the path" limits the power handling capability of the device, since higher power laser signals passing through the adhesive are known in at least certain applications to degrade the adhesive and change its optical properties. Therefore, it would be desirable to avoid or reduce the use of an adhesive in the light path, such as epoxy between optical elements, e.g., epoxy between a filter element and the surface of an optical substrate. In optical multiplexing devices intended for the telecommunications industry, preferably there is as little as possible adhesive in the optical signal path.
It is an object of the present invention to provide improved optical multiplexing devices which reduce or wholly overcome some or all of the aforesaid difficulties inherent in prior known devices. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable and experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain preferred embodiments.